There are many applications in which it is necessary to confirm the identity of an individual. Such applications include the purchase of merchandise using a credit card, cashing cheques at banks or validation of cheques when used in payment for merchandise or service, admission to locations where only authorised personnel are allowed access, and the identification of users of a time-recording system employed for monitoring the times of arrival and departure of staff at a place of work.
In some of these applications it is necessary to ensure that the holder of such a card or any third party into whose hands it may pass as a result of theft or accidental loss, is unable to change the code embodied in or on the card and thereby gain unauthorised access or obtain merchandise dishonestly.
A considerable number of methods have been described for ensuring that the codes embodied in such cards cannot be altered without so drastically mutilating a card that it is no longer capable of being used. Among these methods are several in which the coding is concealed within the structure of the card making it invisible to the naked eye, while it remains detectable by one or more of a variety of techniques depending on, for example, magnetic interaction, radio-frequency coupling, radioactive effects, reflection or attenuation of infrared radiation or other physical phenomena.
A number of techniques have been described in which infra-red radiation is applied to one side of a composite card and a series of infra-red detectors located on the other side of the card respond to the presence or absence of a transmission path through the card at predetermined but visually hidden locations. Some such methods have been disclosed by Scuitto and Kramer in U.S. Pat. No. 3,875,375, by Lawrence Systems Inc. in U.S. Pat. No. 4,066,910, by Interflex Datensystem of Germany in U.K. Pat. No. 2,009,477, by E.M.I. Ltd. in U.K. Pat. No. 1,581,624 and by J. R. Scantlin of Transaction Technology Inc. in U.S. Pat. No. 3,858,032, 3,819,910 and 3,802,101.
Most of the above patents disclosed techniques in which several parallel tracks of data are scanned by a set of several photodetectors, one such track being used as a clock track, while the corresponding data bits on other tracks are either translucent to represent a binary digit ONE or opaque to represent a binary digit ZERO or vice versa.
The mechanisms used to transport the cards past the read-heads, and the parallel signal paths for the several read-heads to the associated digital electronic systems have various levels of complexity according to the details of the intended application.
In U.K. Patent Specification No. 2,108,906, a method has been disclosed of coding a card, in which all the data is located serially along a single track, which includes distinct symbols for both ONEs and ZEROs, and since neither of them is represented by an opaque area, positive clocking is inherent in the single track without any constraint on the rating of scanning. Since the scanning rate is not critical, it is possible to rely on manual presentation of the card to the reader and thereby to simplify the reader mechanism significantly. In the aforesaid U.K. Specification the two distinct symbols consist of windows typically 6 mm long measured across the direction of scan, and of two widths measured along the direction of scan, a width of 0.5 mm typically representing a "ONE" while a width of 0.2 mm typically represents a "ZERO". It has been found that tolerances in production of the windows in practice, while acceptable in the larger apertures are such as to cause problems with maintaining reliability in the smaller windows. It is an object of the present invention to remedy this difficulty.